It has previously been proposed to tap a voltage divider connected across the electrodes of a low-pressure discharge lamp, such as a fluorescent lamp, connect the tap point to a trigger capacitor and to a diac which controls the triggering or firing of the gate electrode of a triac. A terminal of the trigger capacitor is, additionally, connected to one of the main electrodes or main current-carrying path electrodes of the triac.
A circuit of this type is described, for example, in the referenced application Ser. No. 326,498, now U.S. Pat. No. 4,460,848 filed Feb. 12, 1981, by co-inventor Fahnrich hereof, and assigned to the assignee of the present application. The circuit of the referenced application Ser. No. 326,498 is particularly arranged and adapted to start low-pressure discharge lamps with extremely low starting voltages. The standard circuit arrangements for starting utilize a phase controlled starting circuit in which the voltage divider comprises a controllable or specifically set resistor and a capacitor. Phase-controlled circuits, including diacs and triacs, are used in many forms and arrangements to control the speed of motors or the like, for example in small home appliances, in industrial apparatus and the like. The circuit arrangement of the referenced application Ser. No. 326,498 includes a voltage divider and, additionally, a charge resistor and a trigger capacitor in addition to the variable resistor for phase angle control of the circuit. The variable resistor can be used to heat a thermistor used for turn-off of the starter circuit, connected parallel to the charge resistor and the trigger capacitor, in order to assist in the operation of the starting circuit under starting condition. The variable resistor may be replaced by a variable impedance element, for example a control capacitor which is frequency-dependent, or a voltage-dependent resistor. A noise suppression capacitor is connected in parallel to the triac which, in one embodiment particularly adapted for fluorescent lamps which are difficult to start, is formed as a capacitative voltage divider. One of the capacitors is bridged by a self-switching four-layer diode.
The circuit of the referenced application operates satisfactorily; it does require, however, a substantial number of circuit components. If the fluorescent lamp should not fire, the circuit will disconnect automatically when the voltage wave reaches or approaches a maximum. Further attempts at starting thus cannot be initiated inherently by the circuit.